Recently, a ferroelectric thin film element is applied by, using its ferroelectricity, to a thin film capacitor, nonvolatile memory, a piezoelectric actuator using piezoelectricity, or an infrared sensor using pyroelectricity. In these applications, it is required to further improve characteristics of the ferroelectric thin film element. For example, with regard to application to a piezoelectric actuator, it is necessary to enlarge a piezoelectric constant. Also, in the case of a capacitor using a ferroelectric thin film element, leak current and coercive field are required to be at low levels, and deterioration due to reversal of polarization is required to be less.
As a method of manufacturing a ferroelectric thin film element to be applied to a piezoelectric actuator, the following method is disclosed in Japanese Laid-open Patent No.H1-308927. In this disclosure, a magnesium oxide mono-crystalline substrate (MgO substrate) is used. A platinum (Pt) layer oriented by (100) is formed by sputtering on the MgO substrate. Subsequently, a film is formed on the Pt layer by sputtering a target made from sintered material of lead zirconate titanate (PZT) under a condition that a substrate temperature is about 600° C., thereby forming a PZT layer vertically oriented to a film surface. However, in this manufacturing method, an optimum deposition range for obtaining excellent piezoelectricity is narrow, and it is necessary to strictly control film forming conditions. Further, in this manufacturing method, how to further improve piezoelectricity by heat treatment is not described at all.
Also, in application to a nonvolatile memory, a silicon mono-crystalline substrate (Si substrate) capable of forming various semiconductor circuits must be used as the substrate material. Heat treatment is usually executed after film forming in order to form a thin film dielectric element having excellent ferroelectricity on the Si substrate.
For example, in Japanese Laid-open patent No. H6-13565, for manufacture of a ferroelectric memory circuit by using ferroelectric film formed of a PZT layer, a method of improving characteristics of a ferroelectric capacitor by ozone annealing is shown. Ferroelectric film formed by sputtering generally includes defects therein, and if an insulating layer of silicate glass or the like is formed by a chemical vapor-phase method on the ferroelectric capacitor, hydrogen atoms or nitrogen atoms are diffused into the ferroelectric film and turn into substitutionnal impurity atoms, sometimes causing deterioration of ferroelectricity. Ozone annealing is executed to cope with problems of such defects and deterioration, and to improve crystallinity by diffusing active oxygen into a layer which is generated due to decomposition of ozone. In this disclosure, ozone annealing is executed for every film forming process and again after forming ferroelectric film. However, when heat treatment is executed with the ferroelctric film exposed, deterioration or cracking of a surface layer is liable to take place, but a measure for this problem is not described at all.
Also, in Japanese Laid-open Patent No. H7-45475, for example, as for a thin film capacitor with a lower electrode layer, intermediate layer, dielectric layer, intermediate layer, and upper electrode layer formed on a substrate in this order, the intermediate layer is formed from a material containing a metallic element and oxygen. When the dielectric layer is of perovskite-type structure formed from lead (Pb)-magnesium (Mg)-niobium (Nb)-oxygen (O), the intermediate layer is formed from a metallic element including Pb, Mg and Nb, and oxygen (O). This disclosure shows that, by forming an intermediate layer having such composition, it is possible to reduce stress caused due to different thermal expansion coefficients and to prevent generation of cracking or peeling during heat treatment. The intermediate layer has less oxygen (O) at a side coming in contact with the dielectric layer as compared with a side coming in contact with the electrode layer, and is formed by controlling partial pressure of oxygen (O), being a reactive gas, during a film forming process such as sputtering. As for the upper electrode layer side, an intermediate layer is formed on the dielectric layer, and further, the upper electrode layer is formed thereon, which is followed by heat treatment. The purpose of the intermediate layer on the upper electrode layer side is to reduce stress generated due to a difference in thermal expansion coefficients between the upper electrode layer and the dielectric layer. Accordingly, how to improve characteristics of the dielectric layer by executing heat treatment with an intermediate layer formed on the upper electrode layer side is not described at all. Also, when heat treatment is executed after forming the upper electrode layer, it is relatively difficult to sufficiently reduce thermal stress caused due to different thermal expansion coefficients. Therefore, cracking or peeling of the dielectric layer can be prevented but it may be difficult to prevent deterioration of ferroelectricity due to stress induced.